1. Field of the Invention
The present invention relates to a device for modifying the tension of a ribbon wound on a take-up spool or reel, the ribbon being designed, for example, for the thermal transfer, to a medium, of color inks arranged sequentially on the ribbon.
2. Description of the Prior Art
In a known technology of thermal transfer, the ink is not sprayed but deposited and fixed to the printing surface by means of the resistors forming a thermal printer head that heats up a ribbon coated with inks by using a modulated electrical current.
In another known technology, the primary color inks, yellow, magenta and cyan are deposited sequentially on a plastic ribbon making it possible, by the combination of different wavelengths called subtractive wavelengths, to obtain a range of several millions of colors or shades.
To obtain high-definition printing quality, especially in the graphic customizing of memory cards, it is very important to achieve perfect synchronization of the running speed of the ribbon beneath the thermal printer head with the horizontal speed of translation of the card to be printed on beneath the thermal printer.
The ribbon is wound on a feed reel, the shaft of which rotates freely. Another reel, called the take-up reel is used to wind the ribbon after use and is provided with a driving motor means. For a constant linear speed under the thermal printer head, the winding speed, expressed in turns of the take-up reel, varies at each turn since the diameter of the circumference of the winding drum increases with the thickness of a new layer.
The method of hot transfer of inks from the ribbon to the card implies a modulation of the temperature of the thermal head as a function of the shade: the dark colors require higher temperature than the light colors. For the printing of the dark colors, this technical feature tends to cause the clinging of the ribbon to the card owing to the softening of the plastic components of the card, the result of which is printing defects and the additional and unwanted driving of the card by the clinging ribbon, causing other printing defects.
The magnitude of this clinging phenomenon varies according to the composition of the material coating the two faces of the card to be printed. Indeed, in the case of plastic cards, the quantity of plastic components may vary from 5% to 20% depending on the manufacturer, which leads to a variation of the softening temperature from 50.degree. C. to 70.degree. C.
To overcome these drawbacks, use has been made of a flattener device, the function of which consists in modifying the escape angle or angle of release of the ribbon in order to create between the axes of the thermal printer head and the flattener, of a region of lower stress on the ribbon, a sort of belly that delays the pulling of the ribbon during the printing, enabling the spread of the ink in the material of the card, the delay being furthermore profitably used to cool the card and the ribbon.
A device to control the speed of the ribbon is described in the French patent application No. 94 02118 entitled "System and method for controlling the winding of a ribbon on a take-up reel", filed on 24th Feb. 1994 by the present applicant.
However, this prior art device proves to be insufficient when the phenomenon of the clinging of the ribbon becomes more important owing to the quality of the cards and the very dark shades of the inks.
One solution consists in permanently increasing the tension of the winding of the ribbon on the take-up reel but then this tends to cause a compressing of the turns of the ribbon already wound on the reel without its being possible to detect this variable by means of the device described.